I have seen many examples of tiny imitative coins from this time period. In particular, I probably have more examples of mini versions of the Claudius II CONSECRATIO with the altar reverse than official versions. Here's a typical example, about the same size as the OP:
Must have been a pretty wild time in the Gallic Empire.

here's a neat minimi. It's copying an official Constantinopolis with GLORIA EXERCITVS two soldiers reverse. Official examples are pretty small at around 15mm, but this unofficial one is 10.5mm and 0.63gm. It would have been struck around A.D. 335ish.